1. Field of the Invention
The present invention relates to the field of electric motors.
More specifically, the invention concerns an precision high efficiency electric motor with permanent magnets that is able to deliver high torque in extreme operating conditions that do not allow sufficient cooling of the motor's elements by convection to be realized.
2. Description of the Related Art
The technology and operating principles of permanent magnet motors are well known; the main advantage of these motors is that they do not use rotating electrical contacts when only the stator is carrying magnetic induction coils; this has led this type of brush-less motor to be widely used, particularly where the required switching of currents in the coil can be realized using static switches controlled by an ECU.
As shown in FIG. 1, which shows a cross-section of a known permanent magnet motor, the motor 1 comprises a stator 2, fastened to a carrier structure 3 on which are fastened the coils 5 that generate a rotating magnetic field and a rotor 4 on which the permanent magnets 6 are fixed, driven by the stator's rotating magnetic field.
To induce a rotating magnetic field of the stator 2, the current in each coil is itself switched, depending on the position of the rotor 4, to create the torque of the motor 1 by known means, not shown.
This type of motor, which is often used, nevertheless has production constraints where high performance in terms of torque, rotational speed, yield, precision, etc. are required, all the more so when the motor's operating conditions are harsh.
A first limitation of these motors is to realize spaces as small as possible between the rotating parts and the fixed parts to reduce the gaps 8, which can be of less than a millimeter, and to maximize the usable magnetic fields, while preventing any mechanical contact between the rotor 4 and the stator 2.
A second constraint is to limit the energy losses in the motor linked to its operation.
Energy losses by friction are limited by using rollers or bearings 7 suited to the motor's loading.
Energy losses by Joule effect in the conductors of the coils are limited by using a material that is a good electrical conductor, most often copper or aluminum, even though it is not as good an electrical conductor, where the mass of the motor must be reduced.
In addition, since the magnetic field of a coil and the field created by the rotor's permanent magnets, rotating in relation to the stator, are variable, depending on the time at a given point of the stator, these magnetic fields generate induced currents—the Foucault currents—in the conducting elements they go through and, because of the electrical resistance of these conducting materials, they absorb energy which is recovered as heat, via the Joule effect.
The effect of these various sources of energy losses is to increase the temperature of the motor when in operation, which increase in temperature, in addition to the effects of differential thermal expansion of the materials used in the motor's structure that modify the value of the gap 8 and are likely to cause deformations of the motor's structure, also modify the magnetic performance of the permanent magnets, with their magnetization decreasing as the temperature increases.
The effects of this loss of magnetization of the magnets are all the more severe that the magnets with the highest performance are those most sensitive to this temperature effect, which have a relatively low maximum operating temperature, much lower than the Curie temperature, e.g. maximum operating temperatures of the order of 380 Kelvin for neodymium magnets.
To counteract these effects of increasing temperature, it is known to look for improved cooling of the motor in operation.
In ordinary operating conditions, choosing metal cores, which are good heat conductors, for the coils—a mechanically resilient and economical solution—together with forced ventilation of the motor ensure a sufficiently high removal of the heat energy dissipated by the coils and in the metal cores to limit the operating temperature.
To prevent both energy losses by Foucault currents and the corresponding heating up, using an electrically non-conductive material to realize the coil cores is known, in particular a resin-based synthetic material. However, the mechanical performance of such resins are limited and their thermal expansion coefficients are different from those of the motor's metal portions, leading to deformations of the cores when the temperature of the motor increases, which are not acceptable in high-performance motors.
A solution, presented in U.S. Pat. No. 3,974,406 consists of using an electrically insulating and non-magnetic ceramic to realize the core of the stator's coils.
Such non-magnetic and electrically non-conducting ceramics, with a resistivity higher than 10^5 Ohms·m, prevent, like resin-based synthetic materials, losses by Foucault currents, improving the motor's efficiency and limiting its heating.
However, ceramics have a low thermal coefficient of expansion, very different from that of the metallic materials utilized in the other portions of the motor; they are also not ductile and cannot deform as in the case of using synthetic resins, which limits their use to small-size motors, with the risk otherwise of deteriorating the structure of the motor under the effect of the differential expansions.
Because of the limitations and constraints imposed by known solutions, realizing motors able to deliver high torque using high efficiency electric motors using permanent magnets is particularly difficult when the operating conditions preclude the addition of active cooling means, e.g. by forced convection, or when such cooling means are not desirable.
Such conditions are found, in particular, in vacuum, where cooling cannot be achieved by a flow of air and in motors sealed against aggressive conditions: earth, dust, mud, etc.